Thanks Jay Yes there's an optical encoder on the other end of the motor. Depending on the pressure of the roller on the cassette drive wheel it will deform more or less. That effectivley changes the diameter and hence the speed. So you need to set it. Its a DC motor so you can control the speed. Just count the encoder pulses over time and you know what the speed is. Did you glue your pressure hose on or was a push fit enough? Regards Rod On 02/12/2015 15:31, Jay Jaeger wrote:

On Wed, 2 Dec 2015, Paul Koning wrote: I always thought that there was a relatively stable relationship between those! :-) Circumference tends to be a little over 3 times the diameter (3.0 in some states) :-) If it shifts or squeezes out to the sides, not so much. If it compresses, yes. Before TPMS, some cars got a crude estimation of low tire pressure by comparing the RPM of a tire (ABS sensors) with its fully inflated brethren. (circumference changing with pressure change)

Ugh... Could this thread get any more offensive to people's knowledge.. On 12/2/2015 11:13 AM, Tony wrote:

I'm sorry for stirring up this hornet's nest. I actually meant to ask a real question, and the way I phrased it made a mess of things. The real question: for rubber rollers in this sort of application, does the distortion that occurs significantly affect the circumference? Or is the nature of the material such that it's squished out of shape, but circumference does not change much? paul

On Wed, 2 Dec 2015, Paul Koning wrote: Well, I put "emoticons" in, in a futile attempt to indicate that I was joking. ("emoticon captioned for the humo[u]r impaired") I also hoped that the "in some states" would give a further hint to that. I couldn't resist the humour opportunity, but I did not intend it to be nasty - it is a reasonable question. And, without feeling hurt, I can always count on y'all to catch me up on inaccuracies! :-) The question can remain partially inconclusive. If the rubber is shifting without compression, such as if the ID was too large, then the circumference could be partially unaffected, but then the circumference times the RPM of the axis of the roller is no longer the sole determinant of tape motion. Compression will have an effect on EFFECTIVE circumference (the amount the tape gets moved, not the actual measurement around the roller) and remains PI times twice the [compressed] radius of the circular segment at the area of contact. Try to IGNORE the other side of the roller that is not compressed, or imagine if the entire roller were equally compressed, not just the contact patch. That's why I included the explicit example where tire pressure change is/was detected by ABS speed sensors (now often done more directly with TPMS) Here's a similar, but off-topic, one to contemplate: If kids have helium balloons floating against the ceiling of the backseat of your car, which direction do the balloons move when you go around a tight corner?

I say.. come on chaps.. this is not math 101. Any more suggestions for sources of a bit of rubber tube? Rod Smallwood On 02/12/2015 16:13, Tony wrote:

On Wed, 2 Dec 2015, Tony wrote: That's of a CIRCLE, and once you deform it, it ceases to be a circle. I tried measuring a whole bunch of circles, and I can't find any rational reason why dividing the circumference by the diameter never came out even! :-)

On Thu, 3 Dec 2015, Johnny Billquist wrote: OK! I started to wonder whether I needed more bits in my floating point. They just wouldn't resolve in the 24 bits of single precision. Using such a bizarre number is just plain irrational! Seriously, though, in the computer math class, I did make the students manually compute the some bit representations, including PI. I found several students had encountered the same thing that I had run into, of some grade school teachers misinterpreting their textbook and declaring, "PI is about 3.1416 or EXACTLY 22/7"! (insertion of "EXACTLY" was theirs) In fifth grade, I got into a lot of trouble for argueing with that teacher. I think that "tire" is a good term; it implies some of the special issues that come up from compressing from only one side. If you measure the distance from roadway to axle, and imagine a tire that is a perfect circle of that diameter, some of the rotations per mile issues clear up, although obviously not explaining the amount of force required for propulsion relative to what you would need with uncompressible round tire. ("spherical chicken in a vacuum") Tony mentioned selecting based on OD, and then remachining the ID. With my extremely limited machinist skills, I'd be more inclined to look for a match of the ID, and then let elasticity take care of at least part of the OD discrepancy. I've tried to machine rubber. It obviously can be done. But only by somebody more skilled than I. -- Grumpy Ol' Fred cisin at xenosoft.com

On Dec 2, 2015, at 5:22 PM, Fred Cisin <cisin at xenosoft.com> wrote: Howzabout: go to Fort Smith, NT, Canada (or thereabouts, 60? N) Walk or swim as appropriate, measuring distance, due East until you get back to Fort Smith. You got back, so it must have been a circle, yes? Walk or swim to the N. pole, measuring distance again. Compute ratio of distances. I think both Pythagoras and Eratosthenes would be thrilled at the result. - Mark

Groan! -tony

Indeed. And that may even depend on the type of 'rubber' used and how it deforms when pressed against the cartridge drive wheel. Remember that the motpr on these drives is mounted on pivots, which allows it to move back as the catridge is inserted. So a soft rubber roller is going to deform more than a hard one. I disagree with that. The tacho disk is on the bottom of the motor spindle. So the control electronics will count pulses from that and control the motor voltage to get the _motor_ turning at a constant speed. The drive roller in question is between the motor and the tape drive belt in the cartridge (effectively), so while the motor speed might be constamt and correct, the tape speed need not be if the roller is not right. I wondered about using the 'tyres' used on the idler wheels in VCRs, etc (which are still available). Unfortuantely they are sold by the machine they fit, not dimensions, so I don't know if any are a suitable size. And of course the is no shop that still sells things like that (only mail-order :-() so I can't go in and look at them, make measurements, etc. If the O/D is correct then that is all the really matters. If the I/D is different then it would not be hard to turn a different hub. I'll go back to the first question (as I also have 4 of these drive units, 2 in a standalone TU58, 2 in an 11/730 CPU, that need repairing). Does anyone have a UK source of tubing or whatever that will work -tony

On 12/2/2015 9:50 AM, Rod Smallwood wrote: I think I may have glued them, but I am not sure - I do recall that the fit was pretty tight.

I have no idea what the material is called. I just bought something that was close on the O.D. and I.D. that I thought would work. But, when I look up PEX tubing at Home Depot, I found some stuff with .625" O.D. which is maybe just a tad smaller than what I found, and 0.5" I.D. which seems bigger than what I found. Really, the only way I'd know for sure is if I found the same stuff again - and I'm not 100% sure whether I found it at Home Depot or an Ace hardware store. This stuff has an orange jacket, about 17mm O.D., and a concentric inner black part, with an I.D. of 10mm. There is some kind of cloth mesh in between the layers - I can see signs of the pattern in the outer jacket, and I can see threads from it on the drives I actually used it on. I checked, and I don't see any signs of glue - I probably picked it to match the O.D. of the aluminum hub on the drive and press fit it. I certainly sanded the outer jacket down some - but it does not seem to be much at all - perhaps just enough to rough up the surface some. Whatever I did, it ended up without significant flat spots. I seem to recall rotating it between my thumb and forefinger as I rubbed it against some sandpaper. After the repair, I had good luck reading some Micro VAX diagnostic and Microcode tapes. JRJ On 12/2/2015 12:31 PM, Paul Anderson wrote: